Tuberculosis (TB) is the leading cause of death in people living with HIV infection. An increased risk of TB precedes CD4+ T cell depletion and can continue following restoration of T cells by anti-retroviral (ARV) therapy. Treatment of co-infected persons is challenging due to the aggressive course of both individual diseases; a clinical problem that is further complicated by drug interaction and malabsorption issues when combining ARVs and TB chemotherapy. Addressing these clinical challenges requires a much greater understanding of co-infection pathophysiology. A critical knowledge gap has developed in the field due to the limited availability of animal models that support HIV replication. To address this gap we developed a novel model of TB/HIV in the BLT humanized mouse (HuMouse) that reproduces important pathological and immunological features of human disease. By using our unique animal model, we are now able to study the microbial synergy of HIV-1 and Mycobacterium tuberculosis (Mtb) infection in the lung. As a result, we have identified candidate molecular mechanisms for co-infection synergy in lung M? that promote a pro- inflammatory environment and increase mycobacterial proliferation. The objective of this R01 application is to use the new HuMouse model of TB/HIV co-infection to further elucidate the mechanisms whereby HIV infection disrupts the innate immune response of pulmonary M? to Mtb. Our hypothesis is that skewed polarization of pulmonary M? due to HIV-1 infection drives a hyperinflammatory response to Mtb, which in turn produces neutrophil-mediated lung damage, reduced bacterial clearance, and increased tissue necrosis. We have three aims to test the following individual hypotheses that 1) host M? clearance of intracellular Mtb infection is decreased because of competition for L-arginine needed for both nitric oxide synthesis and HIV replication, 2) HIV and Mtb co-infection drive a pro-inflammatory lung environment via inflammasome-dependent activation of IL-1? in host M?, and 3) development of a Th17 cell bias due to the pro-inflammatory environment of co-infection leads to IL-17-dependent neutrophil recruitment and damage in the lungs. These aims will be accomplished by using in vivo studies with Mtb/HIV co-infected humanized mice and in vitro mechanistic studies with isolated lung M?. The expected outcome of our work is a significant advance in our understanding of how HIV manipulates the host inflammatory response to Mtb to promote aggressive disease. The positive impact of these results will be the identification of new targets to intervene clinically to restore normal host immunity and improve bacterial clearance in millions of HIV-infected people.